1. Field of the Invention
The present invention relates to an image pickup apparatus, a method for controlling the image pickup apparatus, and a storage medium. More particularly, the present invention relates to an image pickup apparatus such as a digital still camera and a video camera, a method for controlling the image pickup apparatus, and a storage medium storing a program for controlling the image pickup apparatus.
2. Description of the Related Art
As a method for focusing a camera on a subject by moving a focus lens position, conventional digital still cameras and video cameras employ an automatic focus (AF) method for automatically performing focusing operation by using focus evaluation values generated based on an image signal acquired from an image sensor such as a charge-coupled device (CCD) sensor.
The AF method includes a technique for predicting, while a focus lens is moving, a peak position based on focus evaluation values which have been acquired by a camera microcomputer, and varying the focus lens moving speed based on the predicted peak position.
Specifically, when the focus lens is in proximity to the predicted peak position, it is necessary to acquire sufficient number of focus evaluation values in proximity to the peak position to achieve high-accuracy AF, and therefore the camera microcomputer moves the focus lens at low speed. On the other hand, when the focus lens is far from the predicted peak position, the camera microcomputer moves the focus lens at high speed to achieve high-speed AF.
For example, Japanese Patent Application Laid-Open No. H7-7650 discusses a technique for calculating a ratio of focus evaluation value variation to lens position variation within a predetermined time period as an evaluation value change rate, and varies the focus lens moving speed according to whether the evaluation value change rate is increasing or decreasing.
Japanese Patent Application Laid-Open No. H8-29667 discusses a technique for predicting a peak position based on increasing focus evaluation values at three points and, when the predicted peak position is far from the focus lens position, moving the focus lens at high speed and then moving it at low speed, and re-predicting a peak position.
However, applying a configuration for determining the peak position based on the shape of focus evaluation values as described above to the AF scanning method for acquiring focus evaluation values while driving the focus lens in one direction in a set scanning range may cause a time lag between the time when the camera microcomputer determines that the focus lens is in proximity to the focus evaluation value peak position and the time when speed control is activated.
For example, when focus evaluation values become unstable as in the case of camera shake, angle fluctuation, or low illumination, it becomes difficult for the camera microcomputer to determine the peak position based on the shape of focus evaluation values. In this case, the camera microcomputer cannot determine the peak position before the focus lens comes so close to the focus evaluation value peak position.
If the shape of focus evaluation values cannot be suitably determined in this way, the camera microcomputer cannot complete the processing for reducing the focus lens moving speed until the focus lens reaches the focus evaluation value peak position of the subject, degrading the focus accuracy.
Differences in camera settings such as subject's contrast and frequency, diaphragm, and exposure cause differences in shape of focus evaluation values. Therefore, when determining the peak position based on the shape of focus evaluation values, the camera microcomputer, depending on the subject, cannot complete the reduction of the focus lens moving speed down to a speed for a sampling interval ensuring the AF accuracy until the focus lens reaches the proximity of the peak position.
The above-described Japanese Patent Application Laid-Open No. H7-7650 and Japanese Patent Application Laid-Open No. H8-29667 do not take into consideration a time lag between the time when the camera microcomputer determines the shape of focus evaluation values and the time when speed control is activated. Further, the above-described Japanese Patent Application Laid-Open No. H7-7650 and Japanese Patent Application Laid-Open No. H8-29667 do not change the method for determining the focus evaluation value peak position according to the subject and camera settings. Therefore, depending on the subject and camera settings, the camera microcomputer cannot complete speed control until the focus lens reaches the focus evaluation value peak position, degrading the focus accuracy.